1. Field of the Invention
The present invention relates to a trellis-coded modulation (TCM) decoder and a decoding method thereof, and more particularly, to a TCM decoder for a digital television receiver and a decoding method thereof.
2. Description of the Related Art
A grand alliance (GA)-HDTV which has been developed in the United States in accordance with the study of a large-scale screen and high resolution television, has adopted a vestigial side band (VSB) modulation of a digital transmission method, which is called a GA-VSB system.
One of characteristics of the GA-VSB system selected as the standard for American terrestrial HDTV broadcasting is to employ trellis-coded modulation (TCM) for increasing noise immunity. The TCM which concurrently applies a function of error correction to a conventional modulation technique, can increase transmission efficiency without increase in the bandwidth.
The structure of a TCM coder of the GA-VSB system is shown in FIG. 1A. In the TCM coder, one bit I.sub.2 of 2-bit input I.sub.1 I.sub.2 is input to a convolution coder 106, which has a 2-bit output O.sub.2 O.sub.3, and the other bit I.sub.1 of 2-bit input I.sub.1 I.sub.2 is input to a precoder 100, corresponding to an NTSC rejection filter used for a receiver of the GA-VSB system, which generates a 1-bit output O.sub.1.
If 3-bits O.sub.1 O.sub.2 O.sub.3 are input to a mapper 114, one symbol M.sub.OUT is output. M.sub.OUT has one of 8 levels corresponding to the respective 3-bit sequence O.sub.1 O.sub.2 O.sub.3 as shown in the table of FIG. 1B. Here, delays 104, 108 and 112 indicated by the designation "D", delay input data by a 12-symbol interval and have the same effect as that obtained by parallel-processing 12 coders having a unit delay, which is called 12-symbol interleaving. The 12-symbol interleaving can reduce a burst type noise, and the number of states of the TCM decoder increases when an NTSC rejection filter is used for a receiver, which is one of characteristics of the GA-VSB system.
The Trellis diagram of FIG. 2 shows the state of the TCM coder shown in FIG. 1A. The number of memories, which corresponds to delays, of a convolution coder 106 of FIG. 1A, is two, so that the total state number is four, and the convolution-uncoded bit is one, so that the number of parallel paths indicating the number of transition types is two. For example, in order to transit to the next state 00 when the previous state is 10, input data I.sub.1 I.sub.2 of the coder are 01 and 11, and two types of parallel paths are obtained.
The function of the TCM coder of the GA-VSB system of FIG. 1A is disclosed in Grand Alliance HDTV System Specification, submitted to the ACATS Technical Subgroup, February 1994, but the structure of the TCM decoder has not been disclosed.